I have the same challenge for emailing invoices to contact listed at the Customer and ShipTo level. Both type of contacts are stored and listed in the CustCnt table. For now I will need to create two APR styles with two separate RDDs.
I tried looking for an existing link to the CustCnt table in the out of the box RDD for the ARform but it did not exist.
I even tried giving the second (CustCnt) an alias name but received a server error on save.
I thought about creating an alias table in the same schema for the CustCnt but decided not to change the Erp schema.